My Architect has specified 150mm insulation beneath the entire basement slab, as well as 150mm on all external RC walls to the basement. Is it typical to insulate under a basement slab or is this a complete waste of time?   I understand having to insulate the walls, but the slab seems a little overkill.   There are questions over whether the design will meet the SAP calculations if the under slab insulation is not included.   Slab is 240sqm, so quite big, and with a